1. Field of the Invention
This invention relates to conductive polymer compositions and electrical devices comprising them.
2. Background of the Invention
Conductive polymer compositions and electrical devices such as heaters and circuit protection devices comprising them are well-known. Reference may be made, for example, to U.S. Pat. Nos. 3,793,716, 3,823,217, 3,858,144, 3,861,029, 3,914,363, 4,017,715, 4,177,376, 4,188,276, 4,237,441, 4,304,987, 4,318,881, 4,334,148, 4,388,607, 4,426,339, 4,459,473, 4,514,620, 4,534,889, 4,545,926, 4,560,498, 4,658,121, 4,719,334, and 4,761,541, European Patent Publication No. 38,718 (Fouts et al), and copending, commonly assigned application Ser. Nos. 818,846 (Barma) filed Jan. 14, 1986 now abandoned, 53,610 filed May 20, 1987 (Batliwalla, et al.) now U.S. Pat. No. 4,777,351, 75,929 (Barma et al.) filed July 21, 1987, 189,938 (Friel) filed May 3, 1988, 202,165 (Oswal, et al.) filed June 3, 1988, 202,762 (Sherman, et al.) filed June 3, 1988, 219,416 (Horsma et al.) filed July 15, 1988, and 247,026 (Shafe et al.) filed contemporaneously with this application, the disclosures of which are incorporated herein by reference.
Conductive polymer compositions which exhibit PTC (positive temperature coefficient of resistance) behavior are particularly useful for self-regulating strip heaters and circuit protection devices. These electrical devices utilize the PTC anomaly, i.e. an anomalous rapid increase in resistance as a function of temperature, to limit the heat output of a heater or the current flowing through a circuit. Compositions which exhibit PTC anomalies and comprise carbon black as the conductive filler have been disclosed in a number of references. U.S. Pat. No. 4,237,441 (van Konynenburg et al.) discloses suitable carbon blacks for use in PTC compositions with resistivities less than 7 ohm-cm. U.S. Pat. No. 4,388,607 (Toy et al) discloses appropriate carbon blacks for use in compositions for strip heaters. U.S. application Ser. No. 202,762 (Sherman et al.) discloses the use of semiconductive fillers of relatively high resistivity in combination which carbon black to produce stable conductive polymer compositions with high resistivity. U.S. Pat. No. 4,277,673 (Kelly) discloses self-regulating articles which comprise highly resistive carbon blacks. These blacks, either alone or in combination with a low resistivity carbon black, form PTC compositions which provide significantly shorter annealing times.
As indicated in the references, a large number of carbon blacks are suitable for use in conductive compositions. The choice of a particular carbon black is dictated by the physical and electrical properties of the carbon black and the desired properties, e.g. flexibility or conductivity, of the resulting composition. The properties of the carbon blacks are affected by such factors as the particle size, the surface area, and the structure, as well as the surface chemistry. This chemistry can be altered by heat or chemical treatment, either during the production of the carbon black or in post-production process, e.g. by oxidation. Oxidized carbon blacks frequently have a low surface pH value, i.e. less than 5.0, and may have a relatively high volatile content. When compared to nonoxidized carbon blacks of similar particle size and structure, oxidized carbon blacks have higher resistivities. It is known that carbon blacks which are oxidized provide improved flow characteristics in printing inks, improved wettability in certain polymers, and improved reinforcement of rubbers.